This invention relates to electronic games with programmable sound circuitry. More particularly, it relates to circuitry for filtering digitally generated electronic signals used to synthesize noise and other sounds in amusement games, such as arcade games. Yet more particularly, it refers to circuitry for microprocessor control of sound generation and filtering in amusement games.
Amusement games often have associated sound generating means to add to the enjoyment of playing. The sound generation means may be used to simulate explosion sounds in war-type games, to make noisy sounds to heighten the tension of playing the games, or to provide other sound accompaniment for the games.
Sound generation under microprocessor control is possible with commercially available chips such as the AY3-8910, a programmable sound generator ("PSG"). The PSG has a plurality of output channels and may be used to generate a square wave of a designated frequency in each channel. The PSG is also capable of generating a "noise" signal comprising a frequency modulated pseudo-random pulse width rectangular wave on one or more channels. A capability is provided for amplitude modulating the channel outputs and for mixing the noise signal with a square wave signal in each channel. The PSG output may be used as input to an amplifier driving a speaker system for sound generation. The AY3-8910 is manufactured by General Instrument Corp., Hicksville, N.Y.
There are several difficulties involved in using unfiltered square wave or noise signals, such as those generated by the AY3-8910 as direct input to an audio system. Part of the difficulty lies in the fact that a sequence of square waves has high frequency components which make the resulting sounds harsh and unpleasant to listen to. Secondly, because of the high frequency components when using unfiltered rectangular waves as input, it is difficult or impossible to simulate sounds of explosions which are dominated by low frequency components. Furthermore, many naturally occuring pitched sounds may be best imitated by the additive synthesis technique of combining several channels of sinusoidal (or near-sinusoidal) waves, each tuned to a separate harmonic of the desired fundamental frequency.
Most of the aforementioned difficulties may be overcome simply by the use of programmable electrical filters. For example, a low pass filter with its cut-off set just above the fundamental frequency of a square wave will pass a pure sinusoidal signal of that frequency. Corresponding filters may be utilized to pass the fundamental and a selected number of harmonics. If, however, there are only a limited number of channels of output, as with the AY3-8910, the use of filters of fixed cut-off frequency, or fixed band-pass width in the case of band-pass filters, could only provide a very limited number of possible outputs.
Programmable digital filters are known, but these are relatively expensive and complex because of the number of calculations required. On the other hand, analog filters are relatively simple and inexpensive, primarily because a high degree of precision is not required. It is, therefore, desirable to build filters operating upon conventional analog principles but subject to rapid digital control and using inexpensive components.